An officer of the law is at an extreme disadvantage if he or she cannot see what the passengers are doing after a traffic stop, therefore some states have enacted laws that stipulate the density of after-market tinting. An automobile is not a place to hide in.
It varies by state. In Washington state the back windows have no limits. The front windows can't be more than 35 percent. I have heard that if you have an illegal tint, insurance companies can deny claims if an accident occurs?